Pregnancy alters cardiac receptor afferent discharge in rats

Reflex effects of cardiac receptor (CR) stimulation are attenuated in pregn ant rats. We tested whether CR afferent discharge is reduced during pregnan cy by measuring single fiber activity in response to increases in right atr ial pressure (RAP) in anesthetized pregnant and virgin rats with sinoaortic denervation. Single fiber activity was isolated from fine filaments of the right cervical vagus nerve. Changes in CR discharge, RAP, and arterial pre ssure were recorded in response to atrial saline injections (25-300 mu l). Resting RAP was similar between groups, and spontaneous CR discharge was si milar in pregnant rats (1.95 +/- 0.21 Hz) and in low-frequency (LF) recepto rs in virgin rats (1.30 +/- 0.2 Hz). In virgin, but not pregnant rats, a su bset (24%) of CR had higher-frequency (HF) spontaneous discharge (9.91 +/- 1.19 Hz). During stimulation, the level of RAP above which CR firing increa sed was significantly higher in pregnant rats, but CR activity was clustere d into an LF discharge range. Thus gestation appears to reduce the activity of CR afferents, possibly by increasing stimulus threshold or by selective inactivation of a subset of HF discharging receptors.